Why semen retention is the default way for men

There hasn't been scientific research done yet on the long term benefits of semen retention. That's in part because there it isn't really clear what aspect plays a key role in it for formulating a research question. But that doesn't matter that much for now. You can still speculate about the scenarios logically by formulating a question about it in the following way:

Does is make more sense that semen retention does harm or nothing, or is it the best option?

It is beyond a doubt that it is the best way to go if you simply look at the way humans and most other mammals are made and supposed to operate.

While there are a few studies done that show different outcomes of long term frequent releasing, that doesn't say anything about het opposite.

Semen is not a waste substance like excretions, or what you cough or sneeze out.

There is of course a reason that the possibility of releasing exists in the first place. But a possibility doesn't make it a reason to do it from the perspective of the releaser himself. it's just the possibility in case such a situation occurs for when it's really necessary.

So when you still do release, it goes against the way your body is used to as mentioned above.

Semen retention is not as when you don't poop or don't pee for too long that it will accumulate and cause problems. There's not a single case in history reported.

It also has been shown that semen contains many different important nutrients like zinc. So by regularly releasing you are repeatedly throwing it away, for many men there is actually little to no quantity of these left in the body because of that.

Releasing semen is a very quick way to lose a large amount of those nutrients, compared to getting it from food. And therefore it isn't likely that with regular release the same amount will come each time.

So in conclusion, nothing points in the direction of long term retention being harmful, and only the opposite.